Most discussion of cardiac function has focused on the left ventricle, probably because it comprises most of the heart's mass and is responsible for pumping blood through the aorta to the body. Despite considerable study, however, the quantitative determinants of left ventricular filling pressure and volume, including elasticity and viscoelasticity of the heart muscle, slowed or incomplete relaxation from the previous beat, and negative intraventricular pressures due to systolic shortening to below the ventricle's rest volume, remains undefined. More recently, many people have recognized that factors beyond the left ventricle, including the right ventricle and atria, interact with the left ventricle to determine its function. This interaction is strengthened by the fact that all cardiac chambers are enclosed in the relatively stiff pericardium, so when one or more chambers enlarges it becomes harder for the others to fill. The lungs also affect the heart in several ways: they provide the load against which the right ventricle pumps, they provide the conduit connecting the two ventricles that defines their series arrangement, and they exert an external pressure on the heart, especially during therapeutic maneuvers such as positive pressure respiration. This project will define the relative quantitative importance of these factors throughout the cardiac cycle and relate regional function to the heart's global performance. Experiments will be conducted on dogs using two preparations: (1) Intact animals with radiopaque markers implanted in the heart muscle via a catheter introduced through an artery or vein, and (2) Open chest dogs with surgically implanted ultrasonic crystals and flow probes. The first preparation has the advantage of being more physiologic but has the disadvantage of difficult data acquisition and reduction; the second preparation has the opposite advantages and drawbacks. The results of this project will help define how the heart functions in health and disease and help explain how some treatments work. It relates to physiology, medicine, and engineering.